Mice can come in through a very small hole. If you can stick your pinkie fingertip into a hole, the mouse can squeeze in, too. You have to seal all of the holes thoroughly, using something they cannot easily gnaw through so they cannot gain entry. This includes holes near the floor, as well as near the roof. You also need to make sure the door seals shut, as well as any windows. It may take a little work (OK, it may take a lot of work), but once you get the holes sealed up, the mice will look for lodging somewhere else!